U.S. patent number 6,177,046 [Application Number 09/075,102] was granted by the patent office on 2001-01-23 for superalloys with improved oxidation resistance and weldability.
This patent grant is currently assigned to The Penn State Research Foundation. Invention is credited to George Simkovich, Eric J. Whitney.
United States Patent |
6,177,046 |
Simkovich , et al. |
January 23, 2001 |
Superalloys with improved oxidation resistance and weldability
Abstract
Improved Ni, Fe and Co based superalloys having excellent
oxidation resistance and weldability. The superalloys are obtained
by at least partially replacing the Ni in conventional superalloys
with Pd. The alloys may also contain strengtheners and modifiers
such as Co, W, Mo, V, Ti, Re, Ta, Nb, C, B, Zr, Y, and Hf. The
superalloy has good strength, improved weldability and excellent
oxidation resistance suitable for use in many aerospace and power
generation turbine applications. A preferred embodiment comprises
(in wt %) 1-9% (Al+Ti), 0-0.01% B, 0-0.15% C, 0-25% Co, 5-30% Cr,
0-10% Fe, 0-0.009% (Hf+Y+Sc), 1-15% (Mo+W), 0-8% (Nb+Ta), 40-68%
Ni, 4-32% Pd, 0-10% (Re+Rh), 0-5% V, and 0-0.015% Zr.
Inventors: |
Simkovich; George (State
College, PA), Whitney; Eric J. (State College, PA) |
Assignee: |
The Penn State Research
Foundation (University Park, PA)
|
Family
ID: |
24528657 |
Appl.
No.: |
09/075,102 |
Filed: |
May 8, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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630812 |
Apr 10, 1996 |
|
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Current U.S.
Class: |
420/444; 148/427;
148/442 |
Current CPC
Class: |
C22C
19/03 (20130101); C22C 19/055 (20130101); C22C
19/056 (20130101); C22C 19/057 (20130101); C22C
30/00 (20130101) |
Current International
Class: |
C22C
19/05 (20060101); C22C 30/00 (20060101); C22C
19/03 (20060101); C22C 019/05 () |
Field of
Search: |
;148/408,409,410,419,428,442 ;420/35,95,97,436,440,443,444 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Sheehan; John
Assistant Examiner: Oltmans; Andrew L.
Attorney, Agent or Firm: Monahan; Thomas J.
Government Interests
PRIORITY AND GOVERNMENT SPONSORSHIP
This invention was made with Government support under Contract
Number N00039-92-C-0100 awarded by the United States Department of
the Navy. The Government has certain rights in this invention.
Parent Case Text
This application is also a continuation-in-part of Ser. No.
08/630,812 filed Apr. 10, 1996 now abandoned.
Claims
We claim:
1. A fusion weldable alloy consisting essentially of:
TBL Element Range (wt. %) Al + Ti 1-9 B 0-0.01 C 0-0.15 Co 0-25 Cr
5-30 Fe 0-10 Hf + Y + Sc 0-0.009 Mo + W 1-15 Nb + Ta 0-8 Ni 40-68
Pd 4-32 Re + Rh 0-10 V 0-5 Zr 0-0.015 is the range of 55-72 wt.
%.
2. A fusion weldable alloy consisting essentially of:
TBL Element Range (wt. %) Al + Ti 1-9 B 0-0.01 C 0-0.15 Co 0-25 Cr
5-30 Fe 0-10 Hf + Y + Sc 0-0.009 Mo + W 1-15 Nb + Ta 0-8 Ni 40-68
Pd 4-32 Re + Rh 0-10 V 0-5 Zr 0-0.015 .ltoreq.5 wt % and the total
amount of Pd+Ni lies is the range of 55-72 wt %.
Description
FIELD OF THE INVENTION
The present invention relates to the field of superalloys
containing palladium. The invention is particularly drawn to
nickel-based superalloys useful in aerospace and power generation
turbine applications. The superalloy's weldability, strength and
excellent oxidation resistance properties make it useful in turbine
blade tip manufacturing or refurbishment as well as in other high
temperature components such as combusters, nozzles, flame holders
and seals where these properties are desirable or critical.
BACKGROUND OF THE INVENTION
The term "superalloy" is used to represent complex nickel, iron,
and cobalt based alloys containing additional metals such as
chromium, aluminum, titanium, tungsten, and molybdenum. The term
"based" as used herein means that that element is the largest
weight fraction of the alloy. The additives are used for their high
values of mechanical strength and creep resistance at elevated
temperatures and improved oxidation and hot corrosion resistance.
For nickel based superalloys, high hot strength is obtained partly
by solid solution hardening using such elements as tungsten or
molybdenum and partly by precipitation hardening. The precipitates
are produced by adding aluminum and titanium to form the
intermetallic compound .gamma.' ("gamma prime"), based on Ni.sub.3
(Ti,Al), within the host material.
The properties of superalloys make them desirable for use in
corrosive and/or oxidizing environments where high strength is
required at elevated temperatures. Superalloys are especially
suitable for use as material for fabricating components such as
blades, vanes, etc., for use in gas turbine engines. These engines
usually operate in an environment of high temperature and/or high
corrosiveness. Therefore a need exists for alloys with high
temperature oxidation resistance and/or good hot corrosion
resistance.
Nickel based superalloys are well known in this field. For
instance, U.S. Pat. No. 4,261,742 to Coupland et al. discloses a
superalloy consisting essentially of 5 to 25 wt % chromium, 2 to 7
wt % aluminum, 0.5 to 5 wt % titanium, at least one of the metals
yttrium and scandium present in a total amount of 0.01 to 3 wt %, 3
to 15 wt % in total of one or more of the platinum group metals,
and the balance nickel. The Coupland et al. superalloy has
increased oxidation and hot-corrosion resistance and may be used as
a material for fabricating blades or vanes of gas turbine engines
or components used in coal gasification systems. Also, U.S. Pat.
No. 4,018,569 to Chang discloses an alloy consisting essentially of
8 to 30 wt % aluminum, 0.1 to 10 wt % hafnium, 0.5 to 20 wt % of an
element selected from the group consisting of platinum, rhodium and
palladium, 0 to 3 wt % yttrium, 10 to 40 wt % chromium, and the
balance comprising an element selected from the group consisting of
iron, cobalt and nickel. The Chang superalloy has improved
environmental resistance which may be used to improve the
temperature capability of components in gas turbine engines.
However, neither Coupland et al. nor Chang disclose superalloy
compositions containing palladium in amounts sufficient to improve
the weldability of the superalloy in accordance with the
requirements of the present application. These patents are hereby
incorporated by reference.
Other patents are known that disclose high temperature nickel
containing alloys. Some examples include: U.S. Pat. No. 4,149,881
to D'Silva, U.S. Pat. No. 4,414,178 to Smith, Jr. et al., U.S. Pat.
No. 4,719,081 to Mizuhara, and U.S. Pat. No. 4,746,379 to Rabinkin,
all hereby incorporated by reference. These patents disclose alloys
with various amounts of palladium, chromium and nickel but do not
contain aluminum which is a required element of the present
invention.
Current and next generation turbofan turbine engines use nickel
based superalloys for many of the components in the high
temperature sections of an engine. These sections include the later
stages of the high pressure compressor, the combuster, the high and
low pressure turbine, and the exhaust modules. These components are
subjected to a wide variety of service related degradation
including oxidation, fatigue, creep, corrosion, and erosion. In
nearly all applications, more than one of these phenomena occurs
during turbine engine operation. As a result, alloy design
principally has been concerned with improving the thermomechanical
properties of the alloys. Produceability of the alloy, i.e.,
weldability, castability, forgeability, and machineability are
often considered a secondary or tertiary criterion during alloy
design. However, when weldability is considered during alloy design
the resulting material may be widely used. For example, Alloy 625
and its derivatives (including Alloy 718) are the most widely used
superalloys in the world [H. L. Eiselstein and D. J. Tillack "The
Invention and Definition of Alloy 625", Superalloys 718, 625 and
Various Derivatives, Conference Proceedings, Pittsburgh Pa., June
1991, ed. E. A. Loria].
To improve the oxidation resistance and strength of Ni alloys,
successive generations of alloys have incorporated increasingly
higher levels of aluminum and to a lesser extent titanium. Both Al
and Ti are detrimental to weldability.
There are several modes of cracking that can occur during welding.
One of the most troublesome is strain age cracking of the weld
metal or in the heat affected zone of the base material. Strain age
cracking is the principal reason why nickel based superalloys are
considered to be difficult to weld [Welding Handbook Vol. 4,
Seventh Edition, ed. by W. H. Kearns, p. 233 and 236, .COPYRGT.1982
American Welding Society]. This type of cracking can occur during
cooling from weld temperature, during post weld heat treatment, or
during the application of subsequent weld passes. The primary
reason these alloys exhibit strain age cracking is that the aging
kinetics of the .gamma.' phase is very fast and the alloy can not
accommodate the resulting strain without cracking. FIG. 1 shows the
relationship between an alloy's Al+Ti content and weldability [M.
Prager and C. S. Shira, Weld. Res. Counc. Bul., 128, 1968]. Note
that alloys containing greater than about 3 wt % Al are considered
difficult to weld, in addition as Ti levels increase the allowable
amount of Al present in the alloy also decreases. Also note that
this chart was developed before applicant's discovery of the affect
of the addition of palladium to superalloys, which allows higher
amounts of Al+Ti to be included in the composition at the same
level of weldability. This is discussed more fully below.
For alloys that lie close to the line, such as Rene'41 and
Waspaloy, special heat treatments have been used to reduce
cracking. For example, over aging Rene'41 has been shown to reduce
strain age cracking through the coarsening of the .gamma.' phase
[W. P. Hughes and T. B. Berry, "A Study of the Strain-Age Cracking
Characteristics in Welded Rene'41-Phase 1", Welding Journal, August
1967, p 361-370].
It is common for current generation superalloys to have as much as
12% Al with little or no Ti present. The impossibility of welding
these alloys has a significant impact on the repairability of
components made from such alloys. For example, a turbine blade may
be removed from service due to tip wear while the component still
has a significant portion of its design life remaining. It is
desirable to weld repair the worn area and return the component to
service. Currently these components are repaired using a solid
solution strengthened alloy such as Alloy 625, Hastelloy X, L605,
or HS188. However, these alloys lack the strength and oxidation
resistance of the original material; as a result the repaired
components suffer rapid degradation during subsequent service.
Several other types of cracking can occur in superalloy weldments.
For castings and large grain wrought materials grain boundary
liquation cracking or hot shortness may occur. This type of
cracking is minimized by using a low heat input process such as
laser, electron, or micro plasma arc welding and controlling the
level of carbide forming and impurity elements [T. J. Kelley,
"Welding Metallurgy of Investment Cast Nickel-Based Superalloys",
Weldability of Materials, Conference Proceedings, ed. R. A.
Patterson and K. W. Mahin, .COPYRGT.1990 ASM International]. Also,
weldments can also suffer from nil ductility cracking and restraint
cracking. Both of which are best minimized by proper weld schedule
development and process control.
Current generation Ni based superalloys derive their oxidation
resistance from the formation of an extremely adherent and cohesive
Al.sub.2 O.sub.3 surface layer. The formation of the Al.sub.2
O.sub.3 film depends on the Al content of the alloy and other
elements such as Cr, Y, Hf, and Ti [C. T. Sims and W. C. Hagel,
eds., The Superalloys, .COPYRGT.1972 Wiley, N.Y.]. However,
increasing aluminum content is the most effective method of
improving oxidation resistance. Increasing the aluminum content is
limited by the need to balance other thermomechanical properties.
As a result oxidation resistant coatings have been developed to
increase the Al content at the surface. One technique is to apply a
diffusion aluminide coating where Al is applied by a pack
cementation or a chemical vapor deposition process. Other coating
systems are based on the MCrAlX (M can be Ni and/or Co and X can be
Y and/or Hf) alloys. These alloys are similar to superalloys except
they are very high in Al and contain as much as 1.5% Y or Hf. These
coatings are applied by physical vapor deposition or a thermal
spray process. One variation of the above coating is to
electroplate onto the surface of a component Pd to improve the
oxidation and corrosion resistance [S. Alperine, P. Steinmetz, A.
Friant-Costantini, P. Josso, "Structure and High Temperature
Performance of Various Palladium-Modified Alumined Coatings: A Low
Cost Alternative to Platinum Aluminides," Surface and Coating
Technology, 43/44 (1990), 347-358; P. Lamesle and P. Steinmetz,
"Growth Mechanisms and Hot Corrosion Resistance of Palladium
Modified Aluminide Coatings on Superalloys", Materials
andManufacturing Processes, vol. 10, no. 5, 1053-1075, (1995)].
At Penn State, work has been performed studying the effects of Pd
on the oxidation behavior of Mo--Cr and Mo--W--Cr alloys [D. Lee
and G. Simkovich, "Oxidation of Molybdenum-Chromium-Palladium
Alloys," Oxidation of Metals, 34, Nos. 1/2, (1990); D. Lee and G.
Simkovich, "Oxidation of Mo--W--Cr--Pd Alloys," Journal of Less
Common Metals, 163 (1990), 51-62]. The results show that 1-3 wt.
percent Pd is sufficient to significantly improve the high
temperature oxidation resistance of the alloy systems. The
researchers hypothesized that Pd acts as a Cr reservoir for the
formation of Cr.sub.2 O.sub.3 and as a barrier to the inward
diffusion of oxygen. There have not been previous studies on the
effects that Pd additions have on the oxidation resistance of Ni
based superalloys.
Previous work on platinum additions to superalloys has shown a
beneficial effect on oxidation behavior at high temperature.
Platinum concentrations of about 1-3 weight percent were shown to
significantly reduce the high temperature oxidation rate of the
base metal. The improvement was attributed to an increase in the
diffusion rate of other species [I. M. Allam, H. C. Akuezue, and D.
P. Whittle, "Influence of Small Pt Additions on Al.sub.2 O.sub.3
Scale Adherence", Oxidation of Metals, Vol. 14, No. 6, 1980]. This
may be due to an increase in lattice parameter of the .gamma. phase
caused by the presence of Pt. In the presence of Hf, Pt promotes
inwardly growing Al.sub.2 O.sub.3 pegs that reportedly increased
scale adherence [G. J. Tatlock and T. J. Hurd, "Platinum and the
Oxidation Behavior of a Nickel Based Superalloy",Oxidation of
Metals, Vol. 22, Nos. 5/6, 1984]. It is possible that Pd additions
may also increase oxide scale adherence by the same or other
mechanisms.
The surface segregation of Cr, Pd, Mo, and Ni for a high chromium
ferritic stainless steel has bee studied [W. E. Delport and J. P.
Roux, "The Surface Segregation and Oxidation of Chromium and
Palladium in High Chromium Stainless Steels", Corrosion Science,
Vol. 26, No. 6, pp. 407-417, 1986]. The investigators found that at
550.degree. C. palladium oxidation is virtually complete before the
oxidation of chromium begins. Also, the data suggests that Cr
diffuses more rapidly through PdO than through the bulk material.
This data suggests that the passivation characteristics of a
ferritic stainless steel would be improved if a small amount of
palladium (approximately 0.4 weight percentage) is added to the
steel. Unfortunately, the study did not investigate high
temperatures, where the formation of PdO can not occur.
Gas turbine engines are used in a wide variety of applications
including commercial and military aircraft and for electrical power
generation. Fuel efficiency is a major concern for turbine
manufacturers and operators. Considerable effort is expended during
the design of turbines to improve fuel efficiency over earlier
models, and operators spend a large part of their maintenance
effort to maintain fuel efficiency. Fuel represents a major cost
for both airlines and electric utilities.
Fuel efficiency is increased over earlier engines by incorporating
new designs that take advantage of advances in aerodynamics and
computer simulation. Fuel efficiency is also increased by
incorporating advanced materials that allow the engine to operate
at higher combustion temperatures. Higher combustion temperature
results in more complete burning of the fuel. New materials are
usually more expensive due to an increase in raw material and
manufacturing costs. Often these costs are more than offset by a
decrease in fuel costs. Superalloys have been used extensively in
the hot sections of turbine engines because of their high strength
and excellent resistance to oxidation (usually with the addition of
a coating). Unfortunately superalloys are very difficult to fusion
weld. The inability to fusion weld superalloys results in increased
new part manufacturing cost and an increase in maintenance costs.
It is desirable to develop a new alloy that has both excellent
oxidation resistance and is more weldable than current alloys.
Turbine efficiency is reduced when excessive clearances develop
between rotating components and stator components. In the turbine,
unwanted clearances develop due to the thermomechanical degradation
of the blade tip allowing airflow to leak past the blades. Often
turbine blade tip degradation becomes severe enough for the
operator to remove the blade from service for repair. The repair
consists of welding a sufficient amount of repair material to the
tip and recontouring the blade to final dimensions. The repair
material is often Alloy 625. This material is a solid solution
strengthened nickel alloy that has inferior oxidation resistance to
the original blade material. However, Alloy 625 exhibits excellent
weldability compared to most original blade materials which have
such poor weldability that they can not be used as the repair
material.
Because most repair material, frequently Alloy 625, has poor
oxidation resistance, it does not maintain clearances and causes
the turbine blades to be removed frequently for additional repairs.
By substituting Alloy 625, or another repair material, with the
subject invention, the turbine operator will realize a reduction in
fuel consumption and maintenance costs
SUMMARY OF THE INVENTION
Accordingly, there is a need for a new alloy that provides for
improved weldability while maintaining the oxidation resistance
similar to that of traditional superalloys. The present invention
is a new superalloy with improved weldability, excellent oxidation
resistance and strength adequate for aerospace and power generation
turbine applications. The alloy derives improved weldability, in
part, from the addition of palladium. It is preferred that the
palladium substitute for Ni in conventional type nickel-based
superalloys. The palladium also improves high temperature oxidation
resistance and provides solid solution strengthening.
Palladium additions may improve weldability via four mechanisms:
(1) Pd increases aluminum solubility in the system resulting in a
decrease in the volume fraction of .gamma.', (2) Pd may decrease
the .gamma.' solvus temperature, increase the .gamma.' coarsening
rate and reduce strain age sensitivity, (3) Pd may delay the onset
of .gamma.' precipitation during post weld cooling, and (4) Pd may
increase lattice mismatch in the presence of a species that
exclusively substitutes for aluminum in .gamma.'. The palladium
additions may improve oxidation via the following mechanisms: (1)
Pd will increase the aluminum solubility in the system resulting in
more Al available to form an oxide scale, (2) a Pd enriched layer
will form near the surface increasing the diffusion distance for
other elemental constituents, and (3) Pd may inhibit the diffusion
of oxygen into the substrate thereby reducing internal
oxidation.
One intended use for the alloy is as a filler metal for turbine
blade tip manufacturing or refurbishment. Currently, there is no
superalloy tip material being used as a refurbishment material.
Other high temperature components such as combusters, nozzles, and
seals can also be welded (for new part or refurbishment) using the
new alloy.
Another use of the alloy would be structural components of a
turbine engine, particularly components that require excellent
oxidation resistance and may require repair during the lifetime of
the component. Such a repair may involve welding to restore
dimensional and structural integrity to the part. It would be
important in such a repair that welding does not induce cracks that
may promote early and potentially catastrophic failure.
For the purposes of this invention disclosure the term `welding`
refers to a fusion weld process with or without a filler material.
This type of welding can be performed when dimensional restoration
is required or when piece parts are joined to form an inseparable
assembly.
When used as a repair material for turbine blade tips the new alloy
will save energy by reducing the amount of degradation in
efficiency due to normal operation of a gas turbine (i.e., the
turbine will maintain its designed efficiency for longer periods of
time). Energy will also be saved by allowing the design of turbines
with improved efficiency over those currently available. There is
potential to significantly increase the savings by incorporating
the new alloy into more than one application. Further, energy
savings may be realized when the new alloy is used in other
applications in a turbine.
The new alloys of the present invention provide for weldable
oxidation resistant superalloys that are currently unavailable. The
alloy will allow jet engine manufacturers and overhaulers to
provide improved components at a manufacturing cost similar to
current repair techniques. In addition it will allow components to
be repaired using existing processes. It will also allow the repair
of components with similar oxidation resistance as the original
material so no loss of performance is experienced.
The alloys represent a departure in design philosophy usually
employed in the development of superalloys. Typically weldability
is not a design criterion. In this approach weldability and
oxidation are primary design criteria along with elevated
temperature mechanical properties. It is believed that the
palladium additions will help to achieve the design goals.
These and other advantages of the present invention are
accomplished, in part, by either replacing nickel with palladium in
an existing superalloy or by designing new alloys with palladium as
a major alloying constituent. The new alloy can be based on
conventional nickel, iron, or cobalt based materials, including
superalloys. Further, the new alloy may be an enhancement to
mechanically alloyed or aluminide classes of materials. In its
broadest embodiment, superalloys of the present invention fall
within the scope of the following ranges:
Element Range (wt. %) Al + Ti 0.5-10 B 0-0.01 C 0-0.15 Co 0-25 Cr
5-30 Fe 0-70 Hf + Y + Sc 0-0.009 Mo and/or W 0.5-20 Nb and/or Ta
0-8 Ni 0-70 Pd 2-50 Pd + Ni + Fe 50-72 Re and/or Rh 0-10 V 0-5 Zr
0-.015
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the weldability as a function of aluminum
and titanium content in alloys that do not contain palladium [not
the present invention].
FIG. 2 is a graph showing the effect of solute concentration on the
lattice parameter of gamma nickel.
FIG. 3 shows the effect of solute concentration on the lattice
parameter of Ni.sub.3 Al.
FIG. 4 shows 1150.degree. C. isothermal oxidation results for the
alloys listed in table 5.
FIG. 5 shows 1200.degree. C. isothermal oxidation results for the
alloys listed in table 5.
FIG. 6 shows the 1150.degree. C. isothermal oxidation results for
three alloys with equivalent solute contents and varying Pd
amounts.
FIG. 7 shows the weldability of superalloys as a function of
aluminum and titanium content (atomic %) in superalloys, including
alloys of the present invention and those of the prior art.
FIG. 8 shows the oxidation behavior of Alloys 1, 2, 2 NoPd, and 3
at 1200.degree. C.
FIG. 9 shows isothermal oxidation of Alloy 625 and Pd-modified
Alloy 625 at 1200.degree. C.
DETAILED DESCRIPTION OF THE INVENTION
Pd is a face centered cubic metal and exhibits complete
substitutional solid solubility with Ni [Metals Handbook, Vol. 8,
8th Ed., .COPYRGT. American Society for Metals, 1973; Binary Alloy
Phase Diagrams, Vol. 3, 2nd Ed., T. B. Massalski Ed., .COPYRGT. ASM
Int., 1990]. However, Pd has a higher solid solubility for Al. For
example, Ni has an Al solubility at 1000.degree. C. of 14 atomic
percent while Pd at the same temperature has an Al solubility of 20
atomic percent. Similarly, at 800.degree. C. the Al solubility in
Ni and Pd is 10 and 17 atomic percent respectively.
There is no Al--Pd--Ni ternary phase diagram available, a review of
the binary phase diagrams shows that there is a Pd--Al eutectic at
approximately 23 atomic percent Al that melts at 1055.degree. C. In
the Ni--Al system there is a eutectic at approximately 13 atomic
percent Al that melts at 1385.degree. C. As a result it may be
expected that a ternary eutectic may exist in the 13-23 atomic
percent region and may have a melting point between 1055.degree. C.
and 1385.degree. C. From this information it can be hypothesized
that the .gamma.' aging kinetics may be favorably influenced, i.e.,
decreasing .gamma.' precipitation rate with the addition of Pd.
This would have a favorable impact on a weld to resist strain age
cracking either occurring during post weld heat treatment or in the
weld bead during subsequent weld passes. Thus the addition of
palladium to superalloys has been found by applicant to allow
higher amounts of Al+Ti to be included in the composition at the
same level of weldability.
Table 1 shows the relevant crystallographic data for Ni, Pd, and
Al. Note that the atomic radius and lattice constant are more
closely matched for Pd and Al than for Ni and Al. According to
alloying rules first proposed by Hume-Rothery, the closer the match
between atomic radii the higher the solubility of the solute
[Physical Metallurgy Principles, Second Edition, by Robert E.
Reed-Hill, .COPYRGT.1973 Litton Educational Publishing Inc.]. This
criterion, known as the size factor, states that atomic radii
differences of less than 15% can result in substantial solid
solubility. This limitation is associated with the strain induced
by the solute atoms in the lattice. Note that the difference
between Ni and Al is 13% while the difference between Pd and Al is
only 4%. This supports the conclusions reached by inspection of the
phase diagrams.
TABLE 1 Crystal Structure Of Ni, Pd, and Al Element Crystal Type
Atomic Radius Lattice Constant Ni FCC 1.246 a = 3.5238 Pd FCC 1.376
a = 3.8902 Al FCC 1.432 a = 4.0391 values given in angstroms
The change in .gamma. and .gamma.' lattice parameters as a function
of alloying element has been studied previously [M. Yoshinao, O.
Shouichi, and T. Suzuki, "Lattice Parameters of Ni(.gamma.),
Ni.sub.3 Al(.gamma.'), and Ni.sub.3 Ga(.gamma.') Solid Solutions
with Additions of Transition and B-Subgroup Elements", Acta
Metallurgica, Vol. 33, No. 6, pp. 1161-1169, 1985, .COPYRGT. 1985
Pergamon Press Ltd.]. FIG. 2 shows the effect of solute
concentration on the lattice parameter of gamma nickel. Palladium
has a significant effect on the .gamma.' lattice parameter, of the
elements shown only Nb and Ta have a larger effect. Note that the
solubility limit of each element has not been accounted for in the
figure.
FIG. 3 shows the effect of solute concentration on the lattice
parameter of Ni.sub.3 Al. Pd has a large effect on the Ni.sub.3 Al
lattice parameter. Pd replaces Ni in .gamma.' and in a ternary
Ni--Pd--Al systems partitions equally between .gamma. and .gamma.'.
This results in little net change in the lattice mismatch between
.gamma. and .gamma.'.
The alloying behavior of a variety of Ni.sub.3 X (X can be Al, Ga,
Si or Ge) compound has been investigated [S. Ochiai, Y. Oya, T.
Suzuki, "Alloying Behavior of Ni.sub.3 Al, Ni.sub.3 Ga Ni.sub.3 Si
and Ni.sub.3 Ge", Acta Metallurgica, Vol. 32, 289, 1984, .COPYRGT.
1984 Pergamon Press Ltd.]. Palladium was shown to substitute
exclusively for nickel. At 1050.degree. C. Pd has a solubility in
.gamma.' of approximately 15 atomic percent. In superalloys, cobalt
is the only other common element that was shown to substitute for
nickel, however its .gamma.' solubility decreases with increasing
temperature. Most other elements partition to Al sites or will
substitute for both Ni and Al. For example, Ti and Nb will
partition to the .gamma.' Al sites and result in an increase in
lattice parameter mismatch. Cr will partition to either Ni or Al
sites, however Al sites are more likely to be occupied by Cr.
The addition of palladium will not promote the formation of
topologically close packed phases such as .rho., .mu., or Laves.
This is because palladium has nearly the same electron hole number
as nickel (0.61 for nickel compared to approximately 0.66 for
palladium) [Heat Treatment Structure and Properties of Nonferrous
Alloys by C. R. Brooks, .COPYRGT. 1984 American Society for Metals,
p. 199]. As a result, PHACOMP calculations yield about the same
electron hole numbers for alloys with nickel or palladium. The
formation of TCP phases is not a problem when substituting Ni with
Pd in existing superalloys.
It is difficult to predict the effect palladium will have on the
lattice parameter mismatch, .gamma.' solvus temperature, .gamma.'
coarsening rate, Pd partitioning, aluminum solubility, the
formation of other Pd bearing intermetallics, and the role Pd has
in oxidation. However, the information taken from various binary
and ternary systems indicates that palladium can favorably impact
weldability and oxidation resistance.
If palladium partitions equally between .gamma. and .gamma.', then
there would be little effect on the hardening affect of the
.gamma.'; however, the increase in aluminum solubility should
reduce the total volume fraction present at any given temperature.
If Pd does not partition equally, then the lattice mismatch will
increase and the hardening affect of the .gamma.' will
decrease.
Because of the high solubility of Al in Pd and the presence of a
low melting point Al--Pd eutectic a decrease in the .gamma.' solvus
may occur in complex alloys. This would delay the onset of .gamma.'
precipitation during cooling from welding temperatures. The strains
induced by cooling may be more easily accommodated. Also, the lower
solvus temperature would put more aluminum into solution at
operating temperature.
Although the beneficial effect of Pt on oxidation resistance has
been shown, there are no documented studies on the effect of Pt on
weldability. This is probably because the Pt levels were thought to
be too small to have a measurable affect on weldability. More
importantly, weldability was not a concern for the alloy designers
investigating Pt additions to superalloys.
A review of the Pt--Al phase diagram shows that Pt has less
solubility for aluminum at 1000.degree. C. than either Ni or Pd at
the same temperature [P. R. Hultgren, Selected Values of
Thermodynamic Properties Supp. 1, part 1, Alloys, .COPYRGT. 1963
John Wiley & Sons]. As a result, the solubility of Al in
.gamma. would not increase. This would effectively result in little
or no reduction in .gamma.' volume fraction. Since platinum is also
likely to partition equally between .gamma. and .gamma.' there
would be no net increase in lattice mismatch.
A necessary condition for a species to improve weldability is that
it must replace exclusively Ni in .gamma.' and its solubility for
aluminum must be greater than that of nickel. Pt does not meet the
second condition. Palladium, however, appears to meet the necessary
conditions to improve both oxidation and weldability.
EXAMPLE 1
To improve the weldability and oxidation resistance of an existing
superalloy it is proposed to substitute up to approximately half of
the nickel by weight with palladium. The exact level of
substitution will be dictated by the amount of Pd necessary to
improve weldability. It is likely that the higher the Al+Ti
percentage in the alloy the higher the concentration of Pd
necessary to make a noticeable improvement in weldability. Once the
substitution is made the weight percent of the other constituents
would be adjusted to maintain the same atom proportions as the
original alloy. The following example given in Tables 2 and 3
illustrates the technique for modifying IN738 alloy to one of the
instant invention [Superalloys A Technical Guide, ed. E. F.
Bradley, .COPYRGT. 1988 ASM International]:
TABLE 2 Nominal Composition of IN738 C Ni Cr Co Mo Al B Ti W Zr Ta
Nb w % 0.17 61.3 16 18.5 1.75 3.4 0.01 3.4 2.6 0.1 1.75 0.9 a % 0.8
59.1 17.4 18.2 1.0 7.1 0.05 4.0 0.80 0.06 0.91 0.55
TABLE 3 Nominal Composition of IN738 with Pd Additions C Ni Pd Cr
Co Mo Al B Ti W Zr Ta Nb w % 0.15 36.7 30 13.8 7.4 1.5 2.9 0.01 2.9
2.3 0.09 1.5 0.78 a % 0.8 40.7 18.4 17.4 8.21 1.0 7.1 0.05 4.0 0.80
0.06 0.91 0.55
The amount of Pd necessary to make a noticeable improvement in
IN738 weldability must be determined experimentally. Once the Pd
level has been determined then other thermomechanical, oxidation
and corrosion properties must be determined to establish
suitability for a particular application.
In order to minimize the amount of Pd necessary to achieve the
desired properties a new alloy has been designed. Design criteria
for the new alloy are to maximize weldability and oxidation
resistance while maintaining other properties, such as creep and
rupture, at levels that would meet an intended application. For
example, turbine blade tip repair requires that the tip posses a
minimum rupture strength and some resistance to thermomechanical
fatigue. The present inventors have established a base alloy on
which other alloys can be designed to meet particular needs. The
alloy consists of Ni, Pd, Cr, and Al. Other elements may be added
to increase various thermomechanical properties. Table 4 shows the
limits on the base alloy. It is preferred that the total wt % of
Ni+Pd or of Ni+Pd+Fe (if Fe based) lies within the range of
50-80.
TABLE 4 Limits on Ni Superalloy with Pd ELEMENT WEIGHT PERCENT
Nickel balance Palladium 0.5 to 50 Chromium 0.5 to 30 Aluminum 0.5
to 20
Solid solution strengtheners such as Co, W, Mo, V, Ti Re, Ta, Nb
are added to improve tensile strength. Gamma prime modifiers such
as W, Mo, V, Ti Ta, and Nb are added to improve alloy strength and
creep resistance, especially after an aging treatment. Grain
boundary strengtheners such as C, B, and Zr are added to reduce
grain boundary sliding that may occur during creep. Finally, Y and
Hf are added to improve oxidation behavior, if necessary.
EXAMPLE 2
To improve the weldability and oxidation resistance of existing
superalloys it is proposed to add palladium to the system. Table 5
shows the composition of a test alloy that was based on Alloy 738.
In the test alloy, indicated by Alloy A, Pd simply added to the
base alloy in an amount necessary to achieve approximately 20
atomic percent palladium. All other atom fractions of all other
constituents were then reduced.
TABLE 5 Elements (atomic percent) Al B C Co Cr Mo Nb Ni Pd Ta Ti W
Zr Alloy 738 6.92 0.06 0.47 8.01 17.22 1.02 0.57 59.81 0.0 0.92
4.16 0.8 0.04 Alloy A 5.52 0.05 0.43 6.42 13.82 0.81 0.45 48.05
19.78 0.73 3.31 0.6 0.03
FIG. 4 shows 1150.degree. C. isothermal oxidation results for the
alloys listed in table 5. Note that the base alloy Alloy 738
oxidizes at a significantly faster rate than Alloy A, despite the
reduction of Cr and Al in Alloy A due to the addition of Pd.
EXAMPLE 3
To improve the weldability and oxidation resistance of an existing
superalloys it is proposed to substitute nickel with palladium in
such a way as to maintain the atom fractions of all other elements
in the alloy. This can be accomplished by setting the Ni+Pd
combined atom fraction to a level equal to the atom fraction of Ni
in the base alloy. This technique is illustrated in Table 6. The
atomic percentage of Ni in unmodified Alloy 738 is nominally 67.70.
By adding palladium and keeping the Ni+Pd level equal to 67.70 then
the atom fraction of the remaining constituents will remain
unchanged.
TABLE 6 Constant Atomic Percentage of Solute Elements (atomic
percent) Al B C Co Cr Mo Nb Ni Pd Ta TI W Zr Alloy C 5.56 0.05 0.43
6.43 13.82 .82 0.45 62.68 5.02 0.74 3.34 0.63 0.03 Alloy B 5.56
0.05 0.43 6.43 13.82 .82 0.45 57.70 10.00 0.74 3.34 0.62 0.03 Alloy
A 5.52 0.05 0.43 6.39 13.74 .81 0.45 48.12 19.78 0.73 3.31 0.62
0.03
FIG. 6 shows the 1150.degree. C. isothermal oxidation results for
three alloys with equivalent solute contents and varying Pd
amounts. Note that as Pd levels increase the oxidation rate and
total weight gain decreases. The exact level of palladium
substitution will be dictated by the amount of palladium necessary
to achieve improved weldability and sufficient oxidation
resistance, which is determined experimentally.
EXAMPLE 4
The design and of a new alloy that maximizes the benefits of the
addition of palladium to a superalloy may be the best approach for
newly design components or redesign of existing components. The
design of a new alloy requires knowledge of the intended
application or applications. For gas turbine operating temperatures
between about 430.degree. C. and about 980.degree. C. hot corrosion
may be dominating mechanism of metal attack. Therefore a newly
designed alloy for this temperature range must be resistant to hot
corrosion. Typically chromium is added to alloys to increase hot
corrosion resistance via the formation of a Cr.sub.2 O.sub.3 scale.
To improve weldability of these alloys, palladium is added in an
amount suitable to obtain the desired weldability. Table 7 shows
compositional ranges that would exhibit hot corrosion resistance
and improved weldability. It is preferred that the composition
consists essentially of only these elements. Also, in one preferred
embodiment, the amount of Pt, Hf, Y, and Sc is zero.
TABLE 7 Preferable Most Range Preferable Element Range (wt. %) (wt.
%) Range (wt. %) Al + Ti 0.5-10 1-9 2-5.5 B 0-0.01 0-0.007 0.006 C
0-0.15 0-0.1 0.03 Co 0-25 2-20 3-15 Cr 5-30 10-25 12-22 Fe 0-70
0-30 5 max Hf + Y + Sc 0-0.009 0-0.005 0.005 max Mo and/or W 0.5-20
1-15 1.5-12 Nb and/or Ta 0-8 0-7 0-5 Ni 0-70 10-68 18-63 Pd 2-50
2-45 5-40 Pd + Ni + Fe 50-72 55-70 58-68 Re and/or Rh 0-10 0-5 0.05
max V 0-5 0-0.5 0.1 Zr 0-.015 0-.01 0.005 max
EXAMPLE 5
The design of a new alloy that maximizes the benefits of the
addition of palladium to a superalloy may be the best approach for
newly designed components or redesign of existing components. The
design of a new alloy requires knowledge of the intended
application or applications. For gas turbine operating temperatures
above about 870.degree. C., oxidation is the dominating mechanism
of base metal attack. Therefore a newly designed alloy for this
temperature range must be resistant to oxidation. Typically
aluminum is added to alloys to increase oxidation resistance via
the formation of an Al.sub.2 O.sub.3 scale. To improve weldability
of these alloys, palladium is added in an amount suitable to obtain
the desired weldability. Table 8 shows compositional ranges that
would exhibit oxidation resistance and improved weldability. It is
preferred that the composition consists essentially of only these
elements. Also, in one preferred embodiment, the amount of Pt, Hf,
Y, and Sc is zero.
TABLE 8 Preferable Most Range Preferable Element Range (wt. %) (wt.
%) Range (wt. %) Al + Ti 1-10 3-9 3-7.5 B 0-0.01 0-0.007 0.006 max
C 0-0.15 0-0.1 0.03 max Co 0-20 2-15 3-12 Cr 0-20 2-15 3-12 Fe 0-10
0-5 0.5 max Hf + Y + Sc 0-0.009 0-0.005 0.005 max Mo and/or W
0.5-20 1-18 1.25-15 Nb and/or Ta 0-10 0-8 0-6 Ni 0-70 4-68 12-60 Pd
2-55 3-52 5-45 Ni + Pd 55-72 56-71 57-65 Re and/or Rh 0-10 0-5 0.05
max V 0-5 0-0.5 0.1 max Zr 0-.015 0-.01 0.005 max
EXAMPLE 6
Turbine blade tips are currently repaired using a number of
different processes and materials. Repair cost is of primary
importance to the engine owner. The most cost effective repair is
to use an alloy with excellent weldability and apply a new tip
using a manual tungsten-inert-gas welding process. In some cases, a
more precise welding process such as plasma transferred arc or
laser is used to reduce repair costs. However, as previously
described, alloys with excellent weldability lack strength and
oxidation resistance. In recent years investigators have tried
several methods use advanced alloys as weld fillers. One technique
is to preheat the component to be repaired to very high
temperatures (400-1100.degree. C.). The idea being that the high
temperature preheat will reduce cracking. Although this method has
limited success it suffers from several problems. One problem is
that the high preheat may increase base metal cracking. Another
problem is the cost associated with the preheat. Preheating parts
requires expensive equipment and extra process controls, often
reducing productivity, increasing reject rates. The alloys in this
invention can be used to repair components such as turbine blades,
combusters, seals, vanes, and shafts by conventional repair
procedures. This is advantageous because no additional equipment is
required to use the new alloy. Component repair costs is kept to a
low value.
There are several ways the new alloys can be used for repair. One
way is to use a weld filler alloys that has a composition based on
the original component alloy but modified with Pd (as outlined in
Example 1 and 2). Another way is to use a completely new alloy
based on the compositions (as outlined in Examples 3 and 4).
EXAMPLE 7
To gain additional understanding as to the best compositions for
obtaining both high oxidation resistance and high weldability,
applicant performed additional testing. Four experimental alloys
were fabricated. The nominal composition of each alloy is shown in
Table 9. The weight percentages are shown in parentheses. The only
difference between Alloy 1, 2 and 3 is the amount of aluminum. The
alloys are plotted on FIG. 7 and as can be seen Alloy 1 should be
weldable, Alloy 2 is borderline, and Alloy 3 should be the most
difficult of the three to weld. Alloy 2 NoPd was included as a
baseline for oxidation tests as will be shown later. Alloys 1, 2
and 3 are all within the scope of applicant's invention.
TABLE 9 Composition of Experimental Alloys 1, 2, 2 NoPd, and 3,
Atom Percent Alloy 1 Alloy 2 Alloy 2 NoPd Alloy 3 atom % atom %
atom % atom % Element (wt. %) (wt. %) (wt. %) (wt. %) Al 5 (2.1) 7
(3.0) 7 (3.3) 9 (3.9) Co 10 (9.1) 10 (9.3) 10 (10.2) 10 (9.4) Cr 18
(14.6) 18 (14.7) 18 (16.2) 18 (14.9) Mo 6 (8.9) 6 (9.1) 6 (9.9) 6
(9.1) Nb 1 (1.4) 1 (1.5) 1 (1.6) 1 (1.5) Ni 48 (43.9) 46 (42.5) 58
(58.8) 44 (41.0) Pd 12 (19.9) 12 (20.1) 0 (0) 12 (20.3)
One type of weldability trial performed at Penn State consisted of
a modified circular patch test. The specimen material was Alloy 625
and total sample thickness was 6.35 mm. Testing consisted of a
two-pass laser weld. The first pass fused powder that was
pre-placed in the groove, level with the sample surface. A laser
was used to fuse the pre-placed powder. Powder was then pre-placed
again using a specially constructed tool. Sufficient powder was
pre-placed for the second pass, that after laser fusing a positive
reinforcement was achieved. The height of the build-up was
approximately 0.5 mm above the original substrate.
A 3 kW continues wave Nd:YAG laser was used for all laser weld
trials. In addition to the alloys listed in Table 9 two other
alloys, R'80 and Alloy 625 were tested. R'80 represents a typical
superalloy that has poor weldability and Alloy 625 represents an
alloy that has exceptionally good weldability. Table 10 shows the
nominal compositions of these alloys. It is clear that R'80 is a
gamma prime forming alloy while Alloy 625 is not. This is the
primary reason for the difference in weldability between these two
alloys.
TABLE 10 Composition of R'80 and Alloy 625, Atom Percent R'80 Alloy
625 Element atom % atom % Al 6.35 0 B 0.08 0 C 0.81 0.35 Co 9.2 0
Cr 15.37 24.7 Mo 2.38 5.60 Nb 0 2.25 Ni 58.6 67.10 Ti 5.96 0 V 1.24
0 Zr 0.02 0
The same laser weld parameters were used for the weld trials, i.e.,
they were not optimized for each composition. Further, weldability
is often difficult to determine with a high degree of analytical
accuracy. This is because the formations of cracks in the weld are
dependent not only on the metallurgical aspects of the weld but
also on mechanical considerations, such as weld joint restraint.
Table 11 lists some of the relevant laser weld parameters.
TABLE 11 Laser Parameters for Weldability Trials Parameter Value
Laser Power 3 kW Laser Focus @ focus Laser Type Nd:YAG, CW Powder
Preplaced f-number 16 Shield Gas Ar Travel Speed 20 IPM
After welding, the samples were heat treated by heating the samples
in air to 1100.degree. C. in about 50 minutes, holding for 5
minutes and air-cooling. The purpose of the heat treatment was to
induce cracks due to thermal cycling. The samples were not aged
since the preferred aging temperatures for all the alloys were not
known. Table 12 lists the results of a visual inspection
(10-50.times. magnification) of the weld bead after heat-treating.
Cracking that occurred during the stop/start of the weld was not
included in the analysis since this type of cracking may be
dramatically affected by the weld schedule and no attempt was made
to alter the weld schedule to reduce cracking in the stop/start
region. Because the weld parameters were not refined each
composition and some improvement in welding results is expected
with additional experimentation.
TABLE 12 Weldability Results using Non-optimized Parameters on a
Nd:YAG laser Alloy Type Results, after H.T. Alloy 625 No cracks
Alloy 1 No cracks Alloy 2 NoPd No cracks Alloy 2 No cracks Alloy 3
Cracked R'80 Cracked
Weldability testing was also conducted on Alloys 1, 2, 2 NoPd, and
3 using a carbon dioxide laser and feeding the powder directly into
the beam. Again, welding parameters were not optimized and cracking
in the stop/start of the weld was not included in the analysis. The
results are summarized in Table 13.
TABLE 13 Weldability Results using Non-optimized Parameters on a
CO.sub.2 laser Alloy Type Results, after H.T. Alloy 625 Not tested
Alloy 1 No cracks Alloy 2 NoPd No cracks Alloy 2 No cracks Alloy 3
No cracks R'80 Not Tested
The results of the weldability testing show that Alloys 1 and 2 are
definitely weldable with a minimum of weld parameter development.
Alloy 3 which is considered the most difficult of the three
exhibited crack free welds when welded by experienced personnel. In
conclusion, Alloy 3 is weldable when as predicted by the
information shown in FIG. 7 that it is not weldable. The difference
is applicant's discovery that the addition palladium improves the
weldability of superalloys without sacrificing other desirable
properties of the alloy.
Oxidation Testing on Alloys 1, 2, 2 NoPd, and 3
FIG. 8 shows the oxidation behavior of Alloys 1, 2, 2 NoPd, and 3
at 1200.degree. C. First, compare the difference between Alloy 2
NoPd and Alloy 2. This shows the effect Pd has on oxidation
resistance. Alloy 2 NoPd is by far the worst alloy in oxidation
resistance but the substitution of 12 atomic percent Pd for Ni
(Alloy 2) decreases the oxidation rate dramatically. Second, note
the difference between Alloy 1 and Alloys 2 and 3. This shows the
effect of aluminum on the oxidation resistance. Under these
experimental conditions, Alloy 1 with 5 atomic percent aluminum
oxidized more than Alloys 2 and 3 with 7 and 9 atomic percent
aluminum respectively. These results confirm earlier results on the
role Pd and Al have on the oxidation resistance of alloys
containing both elements.
Oxidation Testing of Alloys Containing Cr and Pd
Pd-modified Alloy 625 mixtures were prepared for oxidation testing.
The composition of the alloys is shown in Table 14. The results of
the oxidation testing are shown in FIG. 9.
TABLE 14 Nominal Composition of Test Alloys Based on Alloy 625
Atomic Percent (wt. %) Alloy Type Cr Ni Mo Nb Pd Alloy 625 24.7
68.25 5.6 1.45 0 (21.5) (67.3) (8.9) (2.3) (0) Alloy 625 + 24.7
67.25 5.6 1.45 1 1%Pd (21.3) (65.7) (8.9) (2.2) (1.8) Alloy 625 +
24.7 65.25 5.6 1.45 3 3%Pd (21.0) (62.8) (8.7) (2.2) (5.77)
It is evident that even small levels of palladium act to retard
oxidation. The data shown in FIG. 9 show that Pd is effective even
in alloys that do not contain aluminum. However, as shown in FIG. 8
Pd and Al have an additive effect on oxidation resistance, which is
why both Pd and Al are required elements in the most preffered
embodiments of the instant invention.
Accordingly applicant has discovered that superalloys within the
compositional ranges expressed below are preferred embodiments of
the invention for the best combination of oxidation resistance and
high weldability.
Element Range (wt. %) Al + Ti 1-9 B 0-0.01 C 0-0.15 Co 0-25 Cr 5-30
Fe 0-10 Hf + Y + Sc 0-0.009 Mo + W 1-15 Nb + Ta 0-8 Ni 40-68 Pd
4-32 Re + Rh 0-10 V 0-5 Zr 0-0.015
Additionally, it is preferred that the wt % of Al is
1.ltoreq.Al<4 and the total amount of Pd+Ni lies is the range of
55-72 wt. %.
In an even more preferred embodiment the compositional ranges fall
within the scope of the following where the wt % of Al is between 2
and 3 and the amount of Ta is .ltoreq.5 wt % and the total amount
of Pd+Ni lies is the range of 55-72 wt %.
Element Range (wt. %) Al + Ti 2-4 B 0-0.006 C 0-0.03 Co 3-15 Cr
10-25 Fe 5 max Hf + Y + Sc 0-0.005 max Mo + W 1.5-12 Nb + Ta 0-7 Ni
45-63 Pd 8-27 Re + Rh 0.05 max V 0.1 max Zr 0-0.005 max
Additionally, it is preferrable that the gamma prime fraction of
these preferred embodiments are .ltoreq.about 45% and even more
preferably .ltoreq.about 35%. At levels above this amount, the
alloys are more susceptible to strain age cracking and are thus not
weldable.
The volume fraction of gamma prime can be determined by gamma prime
extraction, transmission electron microscopy image analysis, and in
certain cases, where the gamma prime particles are large, by
scanning electron microscopy image analysis. Image analysis should
be in accordance with ASTM E562, Standard Test Method for
Determining Volume Fraction by Systematic Manual Point Count. Image
analysis can also be done using an automatic electronic image
analyzer and software provided proper calibration procedures have
been performed. In the case of image analysis, up to 30 different
areas should be evaluated to provide a sound statistical base for
the determination.
Although the invention has been described in detail in the
foregoing for the purpose of illustration, it is to be understood
that such detail is solely for that purpose and that variations can
be made therein by those skilled in the art without departing from
the spirit and scope of the invention except as it may be limited
by the claims.
* * * * *